1. Field of the Invention
The present invention generally relates to offshore vessels used for the production of petroleum products. More specifically, it relates to subsea risers used to connect a Floating Production, Storage and Offloading (FPSO) vessel to flow lines on the seafloor.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A Floating Production Storage and Offloading system (FPSO) is a floating facility installed above or close to an offshore oil and/or gas field to receive, process, store and export hydrocarbons.
It consists of a floater, which may be either a purpose-built vessel or a converted tanker, that is moored at a selected site. The cargo capacity of the vessel is used as buffer storage for the oil produced. The process facilities (topsides) and accommodations are installed on the floater. The mooring configuration may be of the spread mooring type or a single point mooring system, generally a turret.
The high pressure mixture of produced fluids is delivered to the process facilities mounted on the deck of the tanker, where the oil, gas and water are separated. The water is discharged overboard after treatment to eliminate hydrocarbons. The stabilized crude oil is stored in the cargo tanks and subsequently transferred into shuttle tankers either via a buoy or by laying side by side or in tandem to the FPSO.
The gas may be used for enhancing the liquid production through gas lift, and for energy production onboard the vessel. The remainder may be compressed and transported by pipeline to shore or reinjected into the reservoir.
Typically, offshore systems are designed to withstand the “100 year storm”—i.e. the most extreme storm that may statistically be expected to happen once every hundred years at the location where the system is installed. All locations have different hundred year storm conditions, with the worst storms being in the North Atlantic and the northern North Sea. Exceptionally bad storm conditions may occur in hurricane (typhoon) infested areas, but the storm path is relatively narrow (typically about 50 km). Thus, some FPSO mooring systems are designed to be disconnectable, so that the FPSO may temporarily move out of the storm path, and the mooring system need only be designed for moderate conditions.
There are three main types of mooring systems for FPSO vessels:    Spread Mooring wherein the FPSO is moored in a fixed position;    Single Point Mooring (SPM) Systems wherein the FPSO weathervanes around a fixed point; and,    Dynamic Positioning (DP) systems which do not require anchor wires/chains or piled/seabed anchors. This system is the most accurate for station keeping but the most expensive to operate.
A Buoyant Turret Mooring (BTM) system is one type of disconnectable SPM that utilizes a mooring buoy fixed to the seabed by catenary anchor legs and supports crude oil and gas risers—steel or flexible pipes that transfer well fluids from the seabed to the surface. The BTM may be connected by means of a structural connector to the fixed turret. The fixed turret extends up through a moonpool in the tanker, supported on a weathervaning bearing and contains the reconnection winch, flow lines, control manifolds and fluid swivels located above the main deck. The weathervaning bearing allows the vessel to freely rotate about the mooring buoy, in accordance with the prevailing environmental conditions.
The BTM system was developed for areas where typhoon, hurricane or icebergs pose a danger to the FPSO and, primarily for safety reasons, rapid disconnection/reconnection is required. Disconnection and reconnection operations may be carried out from the tanker without external intervention. When disconnected, the mooring buoy sinks to neutral buoyancy under water and the FPSO may sail away.
Another type of FPSO is the Floating Production, Storage and Offloading system for Liquefied Natural Gas (LNG FPSO), a floating facility installed above or close to an offshore gas field in order to receive, process, liquefy, store and export natural gas. It typically consists of a purpose-built floater containing LNG storage tanks with process facilities, gas treatment, liquefaction train(s) and an accommodation block on the deck. The LNG FPSO may be permanently moored to the seabed by a turret-type mooring system.
The high-pressure well stream fluid is delivered from the seabed, via flexible hoses and the swivel, to process facilities on the deck of the LNG FPSO. The process facility, located on the deck, separates the fluid in gas, condensate and water. The water is treated to eliminate any remaining hydrocarbons and discharged overboard. The condensate is treated and stored in separate crude oil tanks.
The gas is separated in methane for LNG production and propane and butane for treatment into LPG. Methane is then treated and liquefied in one or more of the LNG trains which are also located on the deck. The LNG is finally stored at minus 162° C. in the special LNG cargo tanks. On a regular basis the LNG may be transferred from the LNG cargo tanks to LNG shuttle tankers via side-by-side or tandem offloading.
LNG production is by far the largest product on an LNG FPSO, however the FPSO also produces condensate and LPG, which are stored in special LPG and condensate tanks and are offloaded separately via their specific offloading system.
Liquefied Petroleum Gas (LPG) is predominately butane and propane, separated from well fluid stream. LPG may be transported under pressure or in refrigerated vessels (LPG carriers).
A Steel Catenary Riser is a steel pipe hung in a catenary configuration from a floating vessel in deep water to transmit flow to or from the seafloor.
A Single Point Mooring (SPM) is a mooring system that enables the vessel to weathervane whilst it loads or unloads hydrocarbons, chemicals or fresh water. The two categories of SPMs are:                a single point mooring buoy or tower that is designed for use by any trading tanker, and is thus independent of the vessel;        a system, such as a turret mooring, that is incorporated within a vessel such as an FPSO.        
A swivel is a mechanical component consisting of a fixed and a rotating part, connected by means of a roller bearing and a sealing arrangement, allowing fluids to pass between the stationary and the weathervaning part of a Single Point Mooring system.
A swivel stack is an arrangement of several individual swivels stacked on top of each other to allow the continuous transfer on a weathervaning FPSO of fluids, gases, controls and power between the risers and the process facilities on the FPSO deck.
The turret system may be integrated into or attached to the hull of the tanker, in most cases near the bow, and allows the tanker to weathervane around it and thereby take up the line of least resistance to the combined forces of wind, waves and current. A high pressure oil and gas swivel stack is mounted onto the mooring system. This swivel stack is the connection between the risers from the subsea flowlines on the seabed to the piping onboard the vessel. It allows the flow of oil, gas and water onto the unit to continue without interruption while the FPSO weathervanes. For reasons of size and cost, the number of swivels is kept to a minimum, and therefore the flow of oil and gas has to be manifolded in the turret area, particularly when the system produces from a large number of wells.
The turret mooring and high pressure swivel stack are thus the essential components of an FPSO.
U.S. Pat. No. 6,155,193 to Syvertsen et al. describes a vessel for use in the production and/or storage of hydrocarbons, including a receiving device having a downwardly open space for receiving and releasably securing a submerged buoy connected to at least one riser, a rotatable connector for connection with the buoy and transfer of fluids, and a dynamic positioning system for keeping the vessel at a desired position. The vessel includes a moonpool extending through the hull, and the receiving device is a unit which is arranged in the moonpool for raising and lowering, the rotatable connector being arranged at deck level, for connection to the buoy when the receiving unit with the buoy has been raised to an upper position in the moonpool.
U.S. Pub. No. 2013/0299179 describes a riser configuration having a rigid riser portion and a flexible riser portion. The riser configuration also includes a subsea buoy across which the riser portions are connected. Buoyancy means are mounted on the flexible riser portion.
GB2504065 (A) describes a subsea flexible riser used for conveying fluids, such as hydrocarbons. The riser comprises an internal fluid-tight liner, a load-bearing structural layer arranged to withstand internal and external pressure, at least one external load bearing structural layer and an outer protective layer. The internal load bearing structural layer and the internal liner comprise fusible polymer matrix materials and the internal load bearing structural layer is bonded to the internal liner. The internal load-bearing structural layer comprises a fiber reinforced composite material. At least one external load-bearing structural layer is a tensile armor comprising wound metal wires. A method of manufacturing a subsea flexible riser by extrusion is also described.
U.S. Pat. No. 7,766,710 discloses a mooring system that includes a vessel with a lower-side cavity, a turret extending from deck level to the cavity, and a coupling mechanism releasably attaching a mooring buoy to the cavity, at least one buoy-supported riser. The riser end has a coupling member, the riser being slidable via a buoy opening, a riser connector member being attached to a movable transport member upwardly displaceable by a drive element, for when the buoy and vessel are coupled, attaching the riser connector member to the transport member transporting the transport member upward while sliding the riser through the buoy and attaching the coupling member to a vessel transfer duct, and for lowering the riser while sliding the riser through the buoy until the connector member is supported by the buoy, prior to coupling member release, and release of the riser connector member from the transport member, followed by buoy lowering.
U.S. Pat. No. 5,755,607 describes a mooring system for a vessel having a mooring turret which is rotatably coupled to a well of the vessel such that the vessel is free to weathervane about the mooring turret. An anchor leg support base is fixed with the mooring turret with anchor lines secured thereto and anchored to the seafloor. A riser turret is rotatably coupled to the mooring turret and is fixed to the well of the vessel. A riser support base is pivotally coupled to a riser support device for mounting the upper ends of a plurality of flexible risers extending from the seafloor. The riser mounting device is arranged and designed to pivot about two axes at right angles to each other relative to the riser support base. The gimbaled riser mounting device provides a generally uniform load distribution among the risers upon twisting or bundling of the risers which results from weathervaning of the vessel about the mooring turret.
U.S. Pat. Nos. 4,637,335, 4,727,819, 4,802,431, and 5,025,743 disclose a mooring system which can be rapidly installed. The system includes a transfer structure attached to a vessel, an anchor line extending from the transfer structure to a chain table near the seafloor, and catenary chains extending from the chain table to the seafloor. A weight hangs from the chain table to help in setting up the system and in mooring a vessel thereafter. The transfer structure includes a platform that can rotate with respect to the vessel, and a direction sensor for controlling a motor that rotates the platform opposite to rotation of the vessel, to avoid twist of the anchor line.
U.S. Pat. Nos. 4,699,191 and 4,708,178 disclose an improved hose structure for passing fluid across a universal joint, that permits a transfer structure to pivot about two horizontal axes with respect to a vessel or the like at the sea surface. A hose or other flexible conduit has a lower end connected to a pipe on the transfer structure and an upper end connected to a pipe on the vessel which can move up and down and which is biased upwardly. When the transfer structure tilts, to raise or lower the lower end of the hose, the upper end can also rise or fall to minimize bending of the hose, so that a substantially straight hose can be used.
U.S. Pat. No. 4,645,467 discloses an improved offshore terminal of the type that includes a riser loosely anchored at the seafloor so its upper end can extend from a deep underwater level up to the surface to moor a tanker and transfer hydrocarbons to it. A weight hangs from the lower end of the column to improve dynamic mooring and, when the riser is disconnected, to limit the sink depth of the riser. For movement to the deployed position, the riser is lifted by extending a line downwardly from a winch on the vessel, through a central hole in the connector frame down to the top of the riser, the line being pulled to raise the riser until its upper end lies within the central hole of the connector frame. A perforated upper portion of the riser is then in fluid communication with the inner portion of a fluid swivel, so that hydrocarbons can pass out of a conduit within the riser and into the swivel.
EP 0062125 describes a self-standing marine riser which comprises a base, a riser column, a flexible joint between the base and the riser column, and means for providing a loose coupling between the top of the riser column and a vessel, rig or platform on the surface above the location of the riser. The riser column comprises an upper column section which includes at least one buoyancy chamber, and a lower, relatively slender column section The riser includes, or is adapted to support, at least one conduit for the conveyance of a fluid (e.g. oil or gas) or a control line. The buoyancy provided by the upper section of the riser column is preferably variable, and this facilitates the connection and use of the riser. The riser may be used for drilling operations or for production operations. It is said that when employing such a riser, it is not necessary to use large riser tensions in order to maintain the position and structural integrity of the riser in deep water and rough weather.